There are many things that can be confusing about lighting, and particularly about new lighting technologies such as using LED lights for general lighting. I discussed one of these confusing situations in my recent post, "How can 10,000 Lumens be brighter than 20,000 Lumens?" Another question that comes-up periodically is "What is the difference between Lumens and Foot-Candles?" I'll try to bring some clarity to the difference here, along with a little bit about why these measurements (and distinctions) are important.

As mentioned in my previous post, a Lumen is defined as "the International System of Units derived unit of luminous flux, a measure of the total 'amount' of visible light emitted by a source." Part of what is important about this definition is that it focuses on the source of the light. This measurement can be very helpful when comparing different types or kinds of light sources because it provides a standard unit of measurement that directly focuses on the light output, not a secondary measurement, such as the power being used by the light, which is what a description of a light bulb as a "60 watt light bulb" does. However, there are limitations to this measurement because a Lumen measures the light being emitted by the light source, not necessarily how much will be perceived / received / available for use at a location upon which the light is shining.This is one of the areas that a more descriptive measurement such as a Foot-Candle can be useful. A Foot-Candle is defined by Wikipedia as "the illuminance cast on a surface by a one-candela source one foot away." It can also be defined as Lumens per square foot. Though this is similar to a Lumen, as alluded to above, there is a key difference, which primarily focuses on the fact that a Lumen is a measurement of the light emitted from a source, while a Foot-Candle is focused on the amount of light that impacts a surface a given distance away from a light source. Though this can seem like splitting hairs, there is actually good reason for doing this, since there can be significant loss of light as it travels to a surface, and, ultimately, to an observer's eye, due to distance, direction and other factors.Using Foot-Candles to measure levels of illuminance provides an opportunity to compare different light sources in an apples-to-apples manner, even when the two light sources have different characteristics because the focus of the measurement is on the intensity of light at its impact, not at its source. This is particularly important when comparing traditional light sources such as incandescent lights to newer technologies such as LEDs. The reason for this is that the characteristics can be different between two types of lights; for example, lights can emit light in all directions (incandescent), or in a more directional manner (LED), as discussed in more detail in the earlier post that explains How 10,000 Lumens can be brighter than 20,000 Lumens.

Another way to understand the relationship between Lumens and Foot-Candles is to use the metric / International System of Units version of Foot-Candles, which is the measurement Lux. Lux is defined as Lumens per square meter. Therefore, if you already know how many Lumens are emitted by a light source, the Foot-Candles or Lux for that source can be quickly calculated if you know the area over which this light will be shown, in square feet or square meters, respectively. If Foot-Candles are the desired measure, Lux can be converted to Foot-Candles by dividing the Lux by 10.7 to provide the approximate number of Foot-Candles. Whether it is Lux or Foot-Candles, you now have a more precise way of measuring the relative brightness of two different light sources, based on the amount of light that will actually impact a given area. This can be a critical distinction because only light that actually reaches a surface can provide illumination for that surface. If you'd like find our more about topics related to lighting, check out our other blog entries or click on links for our different light products to find out more about the specific lights. As always, if you have any questions, comment below or reach out to us at 619-777-6LED (6533) or through the email link or contact us page.

In an office area, how many foot candles of brightness are needed on a work surface? and how do you calculate this from the output of in lumens from a source.

Much appreciated, Giles

Reply

Brian

2/11/2017 04:57:14 pm

Hi Giles,

You would need to look up the IES recommendation for the particular office you are looking at. You do not calculate the FC's from the source/nominal lumen output. Several factors such as a lens will impact the lumen output coming from a particular source. But more importantly, everyone needs to find out the DELIVERED lumen output of the fixture instead of the NOMINAL/SOURCE lumen output. Nominal or source lumen output is just what it states, the lumen output coming from the source (LED board, bare bulb for incandescent/CFL)....this doesn't take into account any light being lost through a lens or other factors. To get the source/nominal lumen output data, manufacturers of the particular LED's being used (such as Cree, Samsung, etc) will test ONLY the source NOT the source installed into the fixture. For delivered lumen output, find the fixtures IES file which shows where the manufacture tested the fixture with the source installed (the finished product you would purchase at say Lowes or Home Depot). This is the accurate lumen output coming from the fixture. The best way to calculate the FC (foot candle) readings is to take the fixtures IES file and upload it to a lighting design software such as AGI32 or some MFG's place a room estimator on their website for those who don't have the software. I'm sure there's a formula or other way to figure the FC readings by hand...I mean they had to be able to do it back in the day... but I've only used AGI32 (I'm only 26 so I'm not familiar with methods other than lighting software). Hopefully I didn't get too deep but there's no other way to explain this without giving all the details. I'd be happy to comment further as I know this is foreign to many who aren't in the lighting industry.

Reply

MARY KLEE

1/14/2018 11:35:54 am

I am a physical therapist, working in office ergonomics. The usual task lighting required is 30-40 foot candles, and 60-80 foot candles if one is over 50 years of age. I have found it difficult to translate foot candles to watts or lumens, I think because foot candles is a technical term that manufacturers don't use. I measure a client's lighting level using a simple photography light meter I purchased on Amazon. I hope this helps.

Author

Bill Motsko is an Executive supporting Leading Edge Designs (LED). Our team has been providing lighting solutions and helping our clients overcome lights challenges for more than 9 years with a cost-effective, custom approach focused on understanding your needs and providing the most efficient solution. We're passionate about helping to reduce energy costs, while shedding a little light on LED / solid state lighting...